Automated Teller Machines (ATMs) dispense bank notes by picking the required number of notes from an internal store, compiling those notes into a stack and presenting them through the fascia of the ATM for collection by the customer.
In order to ensure the customer can collect all of the bank notes the stack must be sufficiently extended from the fascia to allow the customer to grasp the bank notes, but must still be retained securely by the ATM such that the bank notes do not fall out of the ATM without being grasped by the customer.
During use bank notes become worn, in particular accumulating folds such that they are not flat. As a result worn notes are difficult to handle repeatably and when formed into a stack, the notes may not be well aligned with each other. This may be detected by transport sensors within the ATM which detect that the stack is longer than it should be (i.e. it is longer than the longest note within the stack) and upon detection, the badly aligned stack of notes may be deposited into the purge bin and not presented to the customer. As the purge bin has a limited capacity, repeated rejection of stacks of notes results in the purge bin becoming full. When the purge bin is full, the ATM must be taken out of operation until the purge bin has been emptied.